Carbide-containing bainite in steels

Bainite ordinarily refers to the ferrite-carbide aggregates that form in an intermediate temperature range overlapped by pearlite transformation and proeutectoid ferrite or cementite precipitation at higher temperatures and martensite transformation at lower temperatures. (More recently it has become acceptable to refer to carbide-free bainite when carbide formation is suppressed by alloying.) In the original 1939 paper on high carbon steels by Davenport and Bain, microstructure which corresponds to present 'carbide-containing bainite' was termed 'troostite' when formed just below pearlite transformation temperatures and 'martensite-troostite' when formed at lower temperatures just above the martensite transformation start temperature (M_s). However, it should be mentioned that complexity in morphology of the ferrite and cementite components subsequently led to a wide variety of microstructure definitions proposed by different researchers, sometimes in relation to transformation mechanisms. In this chapter, several kinds of definitions of the carbide-containing bainite structure are reviewed. Then characteristics of each of the ferrite and carbide (mainly cementite) components are described.